This invention relates generally to structural laminates, and more particularly to a technique for producing a contoured laminate formed by thin facings bonded to opposite sides of a core.
While the invention will be described mainly in the context of a contoured laminate formed by aluminum sheet facings bonded to a core of end-grain balsa material, it is to be understood that the invention is not limited to these facings and core materials.
Balsa has outstanding properties unique in the field of lumber, for on the average it weighs less than nine pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength that is superior to any composite or synthetic material of equal or higher density. Balsa is dimensionally stable and may be processed by standard woodworking techniques.
It is known that end-grain balsa wood is capable of supporting far greater loads than flat-grain material of the same density and that low-density balsa in the end-grain direction will support greater loads than flat-grained material of higher density. The cellular structure of balsa is such that the number of cells per cubic foot is extremely high, the wall thickness of each cell being quite thin. The cells are effectively independent of each other, each cell being comparable to an independent column or fiber. The fibers of balsa wood are substantially parallel to each other.
Structural sandwich laminates can be created by bonding thin facings or skins to balsa wood panels which function as a core. Thus the Kohn et al. U.S. Pat. No. 3,325,037 and the Lippay U.S. Pat. No. 3,298,892 disclose structural sandwich laminates whose core is formed of end-grain balsa, the resultant laminates having a remarkably high strength-to-weight ratio as well as excellent thermal insulation properties.
End-grain balsa-core sandwich laminates are widely used in transportation and handling equipment, such as for floors of railroad cars, shipping containers, cargo pallets, bulkheads, doors and reefer bodies, as well as in a variety of other applications. These laminates are also employed for structural insulation in aircraft applications, in housing and in boating.
In some applications, the need exists for a contoured balsa core laminate having thin metal facings, the curvature being appropriate to that of a boat hull, a tank or other shaped article. The use of contoured aluminum sheeting to form canoes and other small boats is well known. Because a thin aluminum hull is lacking in strength and stiffness, reinforcement expedients such as ribs must be used. This complicates the manufacture of the boat, it adds to the cost thereof and results in a small boat interior having obstructions.
Moreover, a hull made entirely of aluminum has other undesirable properties; for when the boat is driven by an outboard motor, the metal hull acts as a resonator and picks up and effectively amplifies motor noise and vibration. This not only results in discomfort to the passengers of the boat, but when the boat is used for fishing, the noise is transmitted to the surrounding waters and tends to drive away the fish.
The drawbacks incident to aluminum and other metal hulls could, in theory, be obviated by a laminate sandwich structure using a balsa core bonded to thin aluminum facings. This structure would inherently possess far greater strength than a non-laminated hull and have the added advantage of being relatively quiet, for a balsa wood core affords acoustic damping. Also, the balsa core would impart buoyancy to the boat hull.
And while one can produce a balsa core-aluminum facings laminate in the manner disclosed in the above-identified copending patent application, this laminate cannot be contoured to conform to a hull or any other curved form. Should one bond aluminum facings to an end-grain balsa core and then seek to form the laminate to cause it to assume a predetermined contour, the balsa core would rupture in the course of bending.
End-grain balsa is composed of a myriad of parallel fibers, and should these fibers be bonded at one end to a planar metal facing and at the other end to a parallel planar facing, the resultant rigid structure would be highly resistant to deformation. Should the planar laminate then be subjected to heavy bending stresses, the balsa core would be ruptured in the forming process.